AqBanking

/bin/sh ../../libtool  --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -DBUILDING_GTK3_GUI -DPACKAGE=\"gwenhywfar\" -I. -I../..  -I../../gwenhywfar5 -I/usr/include/gtk-3.0 -I/usr/include/pango-1.0 -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/glycin-2 -I/usr/include/at-spi2-atk/2.0 -I/usr/include/at-spi-2.0 -I/usr/include/atk-1.0 -I/usr/include/dbus-1.0 -I/usr/lib/x86_64-linux-gnu/dbus-1.0/include -I/usr/include/fribidi -I/usr/include/pixman-1 -I/usr/include/harfbuzz -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/gio-unix-2.0 -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/sysprof-6 -pthread -fvisibility=hidden   -O2 -pipe -g -Wp,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer  -Wall -Wall -Wall -MT gtk3_gui.lo -MD -MP -MF .deps/gtk3_gui.Tpo -c -o gtk3_gui.lo gtk3_gui.c
libtool: compile:  gcc -DHAVE_CONFIG_H -DBUILDING_GTK3_GUI -DPACKAGE=\"gwenhywfar\" -I. -I../.. -I../../gwenhywfar5 -I/usr/include/gtk-3.0 -I/usr/include/pango-1.0 -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/glycin-2 -I/usr/include/at-spi2-atk/2.0 -I/usr/include/at-spi-2.0 -I/usr/include/atk-1.0 -I/usr/include/dbus-1.0 -I/usr/lib/x86_64-linux-gnu/dbus-1.0/include -I/usr/include/fribidi -I/usr/include/pixman-1 -I/usr/include/harfbuzz -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/gio-unix-2.0 -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/sysprof-6 -pthread -fvisibility=hidden -O2 -pipe -g -Wp,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -Wall -Wall -Wall -MT gtk3_gui.lo -MD -MP -MF .deps/gtk3_gui.Tpo -c gtk3_gui.c  -fPIC -DPIC -o .libs/gtk3_gui.o
libtool: compile:  gcc -DHAVE_CONFIG_H -DBUILDING_GTK3_GUI -DPACKAGE=\"gwenhywfar\" -I. -I../.. -I../../gwenhywfar5 -I/usr/include/gtk-3.0 -I/usr/include/pango-1.0 -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/glycin-2 -I/usr/include/at-spi2-atk/2.0 -I/usr/include/at-spi-2.0 -I/usr/include/atk-1.0 -I/usr/include/dbus-1.0 -I/usr/lib/x86_64-linux-gnu/dbus-1.0/include -I/usr/include/fribidi -I/usr/include/pixman-1 -I/usr/include/harfbuzz -I/usr/include/freetype2 -I/usr/include/libpng16 -I/usr/include/gio-unix-2.0 -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/sysprof-6 -pthread -fvisibility=hidden -O2 -pipe -g -Wp,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong -grecord-gcc-switches -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -Wall -Wall -Wall -MT gtk3_gui_dialog.lo -MD -MP -MF .deps/gtk3_gui_dialog.Tpo -c gtk3_gui_dialog.c  -fPIC -DPIC -o .libs/gtk3_gui_dialog.o
gtk3_gui_dialog.c:66:10: fatal error: w_hsplitter.c: No such file or directory
   66 | #include "w_hsplitter.c" 
      |          ^~~~~~~~~~~~~~~
compilation terminated.
make[3]: *** [Makefile:692: gtk3_gui_dialog.lo] Error 1

Looks like you forgot to add it to EXTRA_DIST in gui/gtk3/Makefile.am.